The present invention relates to a double conversion television tuner and, more particularly, to a double conversion television tuner which eliminates the need for the frequency converter of a digital signal processing block during selective reception of a television broadcast with its base-band signal in either analog or digital format, whereby both the deterioration in the S/N ratio and the generation of signal distortion attributable to beat disturbances are minimized.
Along with significant advances in multimedia-related technologies in recent years has come an accelerated transition from analog broadcast which uses an analog signal as its base-band signal to digital broadcast which utilizes a digital base-band signal in a variety of telecast operations including terrestrial television, cable television (CATV) and satellite broadcasting television (BSTV).
In such telecast operations, digital broadcast differs from analog broadcast not only in the base-band signal format but also in modulation method. For example, digital broadcast typically employs QPSK (quadrature phase shift keying) or QAM (quadrature amplitude modulation). With this modulation scheme, the television tuner in digital television sets comprises an analog and a digital signal demodulation circuit for demodulating the received signals. As such, the television tuner is called a double conversion television tuner.
FIG. 4 is a block diagram of a conventional double conversion television tuner. As shown in FIG. 4, a conventional double conversion television tuner 41 comprises an antenna input terminal 42, a broad-band pass filter 43, a first frequency conversion stage 44, a second frequency conversion stage 45, a signal divider 46, an analog signal modulator 47, a third frequency conversion stage 48, a digital signal demodulator 49, an analog signal output terminal 50, a digital signal output terminal 51 and a channel selector (channel-select buttons) 52.
The first frequency conversion stage 44 includes a first frequency mixer 53, a first oscillator 54, a phase-locked loop (PLL) 55 and a first intermediate frequency signal selection band-pass filter 56. The second frequency conversion stage 45 has a second frequency mixer 57, a second oscillator 58 and a second intermediate frequency signal selection low-pass filter 59. The third frequency conversion stage 48 comprises a third frequency mixer 60, a third oscillator 61 and a digital signal selection low-pass filter 62.
In the first frequency conversion stage 44, the first frequency mixer 53 has its first input terminal connected to the antenna input terminal 42 via the broad-band pass filter 43, its second input terminal connected to the output terminal of the first oscillator 54, and its output terminal connected via the band-pass filter 56 to the first input terminal of the second frequency mixer 57 in the second frequency conversion stage 45 located downstream. The phase-locked loop 55 has its input terminal connected to the output terminal of the first oscillator 54, its output terminal connected to the control input terminal of the first oscillator 54, and its control input terminal connected to the output terminal of the channel selector 52.
In the second frequency conversion stage 45, the second frequency mixer 57 has its second input terminal connected to the output terminal of the second oscillator 58, and its output terminal connected via the low-pass filter 59 to the input terminal of the signal divider 46 located downstream. The analog signal demodulator 47 has its input terminal connected to the first output terminal of the signal divider 46, and its output terminal connected to the analog signal output terminal 50.
In the third frequency conversion stage 48, the third frequency mixer 60 has its first input terminal connected to the second output terminal of the signal divider 46, its second input terminal connected to the output terminal of the third oscillator 61, and its output terminal connected via the low-pass filter 62 to the digital signal demodulator 49 located downstream. The digital signal demodulator 49 has its input terminal connected to the output terminal of the low-pass filter 62, and its output terminal connected to the digital signal output terminal 51.
The double conversion television tuner 41 of the above-described constitution works substantially as follows: a desired television broadcast is initially selected by operation of the channel selector (channel-select buttons) 52. In response to the selection, the output voltage of the phase-locked loop 55 varies. This in turn alters the frequency of a first local oscillation signal of the first oscillator 54 whose frequency is controlled by the output voltage of the PLL 55. At this point, the signal received via an antenna (not shown) is forwarded to the antenna input terminal 42. The broad-band pass filter 43 is given the received signal and removes unnecessary components therefrom. From the broad-band pass filter 43, the received signal is sent to the first frequency conversion stage 44. The first frequency conversion stage 44 mixes the received signal and the first local oscillation signal in terms of frequency, and supplies the frequency-mixed output to the band-pass filter 56. From the received signal, the band-pass filter 56 extracts only the channel-select received signal selected by the channel selector 52. The signal thus extracted is a first intermediate frequency signal obtained by converting the received signal into a first intermediate frequency f.sub.IF1. The first intermediate frequency signal is fed to the second frequency conversion stage 45 located downstream.
The second frequency conversion stage 45 mixes in terms of frequency the first intermediate frequency signal and a second local oscillation signal which is generated by the second oscillator 58 and which has a fixed frequency. The frequency-mixed output is sent to the low-pass filter 59. The low-pass filter extracts as a second intermediate frequency signal the channel-select received signal obtained by converting the received signal into a second intermediate frequency f.sub.IF2. The second intermediate frequency signal is supplied to the signal divider 46 located downstream. The signal divider 46 divides the second intermediate frequency signal into two parts, one part destined to the analog signal demodulator 47 and the other part to the third frequency conversion stage 48.
If the base-band signal of the second intermediate frequency signal thus supplied is an analog signal, the analog signal demodulator 47 demodulates the second intermediate frequency signal to analog format and sends the demodulated output to the analog signal output terminal 50. The third frequency conversion stage 48 mixes in terms of frequency the second intermediate frequency signal and a third local oscillation frequency generated by the third oscillator 61, and sends the frequency-mixed output to the low-pass filter 62. The low-pass filter 62 extracts as a third intermediate frequency signal the channel-select received signal obtained by converting the received signal into a third intermediate frequency f.sub.IF3. The third intermediate frequency signal is fed to the digital signal demodulator 49 located downstream. At this point, if the base-band signal of the supplied third intermediate frequency signal is a digital signal, the digital signal demodulator 49 demodulates the third intermediate frequency signal to digital format, and sends the demodulated output to the digital signal output terminal 51.
In the above-described double conversion television tuner 41 of the prior art, the first frequency conversion stage 44 converts the frequency of the channel-select received signal to the first intermediate frequency f.sub.IF1 which is higher than the frequency of the channel-select received signal, and the second frequency conversion stage 45 converts the first intermediate frequency f.sub.IF1 to the second intermediate frequency f.sub.IF2 which is lower than the first. Where the base-band signal is a digital signal, the tuner 41 further needs for demodulation purposes a frequency still lower than the second intermediate frequency f.sub.IF2. This requires the third frequency conversion stage 48 to convert the second intermediate frequency f.sub.IF2 to the third intermediate frequency f.sub.IF3 which is lower than the second.
As outlined, the conventional double conversion television tuner 41 needs three frequency conversion stages: the first frequency conversion stage 44, the second frequency conversion stage 45 and the third frequency conversion stage 48. The three-stage structure involves installing the first oscillator 54 for generating the first local oscillation signal, the second oscillator 58 for generating the second local oscillation signal and the third oscillator 61 for generating the third local oscillation signal. The three-oscillator setup is vulnerable to beat disturbances between oscillators and can contribute to deteriorating the reception performance of the double conversion television tuner. Another disadvantage of the conventional constitution is that it needs numerous circuit components and thereby pushes up the production costs of the double conversion television tuner.